1. Field of the Invention
The present invention pertains to the filtration of particulates from a gas stream by means of a filter cake composed of particulates previously drawn from the gas stream.
More particularly, the present invention pertains to method for filtering a particulate-laden gas stream by forming a filter cake comprising particulates drawn from the particulate-laden gas stream, the porosity of the filter cake being such as not to impede the flow therethrough of the gas stream.
2. State of the Art
A technique for electrostatically attracting charged particulates from a gas stream onto metallic collecting structures was described in U.S. Pat. No. 1,931,436 to Walther Deutsch. The metallic collecting structures described by Deutsch were configured so as to maximize the surface areas providing deposition sites for the charged particulates.
In the Deutsch patent, the primary filtration mechanism was the deposition of the particulates directly on the metallic collecting structures. According to the technique described by Deutsch, the particulates deposited on the collecting structures were continually removed therefrom in order to maximize the metallic surface area exposed to the charged particulates in the gas stream.
The use of a cake of particulates drawn from a gas stream as the primary filter medium for removing subsequent particulates from the gas stream was described in U.S. Pat. No. 3,910,779 to Gaylord W. Penney. According to the technique described by Penney, the particulates in the gas stream were electrically charged in a corona region, and were then carried by the gas stream to a separate filtering region downstream of the corona charging region. In the filtering region, a textile fabric filter was mounted on a metalic support structure, with a non-corona electric field being maintained at the collecting surfaces of the filter. According to Penney, no corona should be permitted near the particulate collecting surfaces of the filter.
There was no suggestion in the prior art, however, to use a filter cake formed by the deposition of electrostatically attracted charged particulates directly onto an apertured collecting structure as the primary filter medium for removing subsequent particulates from a gas stream, where the size of the apertures of the collecting structure is more than an order of magnitude larger than the average size of the particulates to be removed from the gas stream.
Where screens of other foraminous collecting structures were used in the prior art to collect electrically charged particulates drawn from a gas stream, the consensus of practitioners skilled in the art was that the apertures of the collecting structures must be not more than an order of magnitude larger than the average size of the particulates to be collected.